Most organic polymeric materials undergo some degradation when exposed to the high energy photons of ultraviolet radiation. The degradation manifests itself, depending on the polymeric material, in yellowing, discoloration, embrittlement and other loss of physical properties. Polycarbonate resin is no exception and it is, therefore, an object of this invention to provide a method of producing a polycarbonate resin article which is highly resistant to ultraviolet radiation degradation.
Ultraviolet radiation absorbers have been used with various resins, such as polycarbonates, acrylics, polyesters, polyolefins, vinyls, and polystyrene to provide protection against attack by ultraviolet radiation. The ultraviolet radiation absorber generally has a very high UV absorptivity relative to that of the polymer and functions by screening out the damaging ultraviolet portion of light. To qualify as a successful ultraviolet light absorber for a polymer, particularly for polycarbonate, the absorber should fulfill several requirements. The absorber advantageously has a high specific absorptivity in the range of wavelengths that are most deleterious to the polymer and that are present in the source of the exposure. The absorber should also be compatible with the polymer and not substantially deleteriously affect its physical properties. For most applications, the absorber preferably does not significantly absorb light in the visible region of the spectrum, or a color will be imparted to the polymer. Stabilization of polymers against ultraviolet radiation has typically been accomplished by blending effective amounts of UV absorber with the polymer prior to forming operations. Recently, a technique has been developed which involves impregnating the surface of the polymer with a UV absorber. For example, U.S. Pat. No. 4,322,455 to Olson discloses a process for producing an ultraviolet stabilized polymeric, particularly polycarbonate, article comprising impregnation of the surface of the article with an ultraviolet radiation absorber by heating the polymeric article and applying onto the surface of the heated article an ultraviolet radiation absorbing compound dissolved in a nonaggressive carrier.
Surface impregnation of polymers has several advantages over incorporation of the UV absorber throughout the polymer matrix. Generally, less UV absorber is required to achieve the same or greater degree of polymer stabilization. Since the UV absorber resides only at the surface of the polymer, the effects of the UV absorber on the physical properties of the polymer are minimized. Moreover, surface impregnation temperatures are generally milder than those encountered during forming operations such as extrusion or injection molding. Therefore, thermal degradation of the UV absorber is less of a concern with surface impregnation techniques.
Nevertheless, current surface impregnation procedures have been attended by certain disadvantages. Because the UV absorber is applied in solution to the polymer surface, relatively low molecular weight compounds, which are soluble in the solvent employed, have typically been used. The polymeric materials often are subjected to elevated temperatures subsequent to surface impregnation. These temperatures may be encountered, for example, in thermoforming operations or in high temperature applications of the polymer product. Conventional UV absorbers that have been applied by surface impregnation often are somewhat volatile at such temperatures. Substantial amounts of the UV absorber can be lost through volatilization at elevated temperatures. Accordingly, a need exists for a surface impregnation, non-volatile UV absorber which can be impregnated into the surface of a polymer yet not substantially alter the physical properties of the polymer.